An 18-year-old Denver woman was among five people killed when a gunman opened fire at a nightclub in a popular Mexican resort, her brother said Monday.

Roberto Martinez told NBC News that his sister, Alejandra Villanueva, was killed in the attack at the Blue Parrot nightclub in Playa Del Carmen at around 2:30 a.m. (3:30 a.m. ET) on Monday.

The gunman allegedly opened fire after being denied entrance to the BPM electronic music festival taking place inside.

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Five dead, 15 hurt in Mexican nightclub shooting

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Five dead, 15 hurt in Mexican nightclub shooting

Mexican police agents patrol near a nightclub in Playa del Carmen, Quintana Ro state, Mexico where 5 people were killed, three of them foreigners, during a music festival on January 16, 2017.
A shooting erupted at an electronic music festival in the Mexican resort of Playa del Carmen early Monday, leaving at least five people dead and sparking a stampede, the mayor said. Fifteen people were injured, some in the stampede, after at least one shooter opened fire before dawn at the Blue Parrot nightclub during the BPM festival.
/ AFP / STR (Photo credit should read STR/AFP/Getty Images)

Mexican police agents, investigate at a nightclub near the beach in Playa del Carmen, Quintana Roo state, Mexico where 5 people were killed, three of them foreigners, during a music festival on January 16, 2017.
A shooting erupted at an electronic music festival in the Mexican resort of Playa del Carmen early Monday, leaving at least five people dead and sparking a stampede, the mayor said. Fifteen people were injured, some in the stampede, after at least one shooter opened fire before dawn at the Blue Parrot nightclub during the BPM festival.
/ AFP / STR (Photo credit should read STR/AFP/Getty Images)

View of the Blue Parrot nightclub in Playa del Carmen, Quintana Ro state, Mexico where 5 people were killed, three of them foreigners, during a music festival on January 16, 2017.
A shooting erupted at an electronic music festival in the Mexican resort of Playa del Carmen early Monday, leaving at least five people dead and sparking a stampede, the mayor said. Fifteen people were injured, some in the stampede, after at least one shooter opened fire before dawn at the Blue Parrot nightclub during the BPM festival.
/ AFP / STR (Photo credit should read STR/AFP/Getty Images)

A shoe remains in the beach near a nightclub in Playa del Carmen, Quintana Roo state, Mexico where 5 people were killed, three of them foreigners, during a music festival on January 16, 2017.
A shooting erupted at an electronic music festival in the Mexican resort of Playa del Carmen early Monday, leaving at least five people dead and sparking a stampede, the mayor said. Fifteen people were injured, some in the stampede, after at least one shooter opened fire before dawn at the Blue Parrot nightclub during the BPM festival.
/ AFP / STR (Photo credit should read STR/AFP/Getty Images)

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Martinez said his sister was born in California but raised in Colorado. He said she was an art lover and described her as "magnificent" during an interview with NBC station KUSA.

"I'm pretty much devastated, I was in shock all day," he said. "I didn't want to think that it was true, that my sister was gone. Until I started receiving calls, after calls, after calls saying they were sorry ... that's when it started kicking in that I wasn't going to see her anymore."

Villanueva had just graduated high school and was going to community college, according to her brother. He said she planned to become a teacher for children with Down syndrome.

"She loved art, she loved drawing, she was magnificent," he said. "She did a couple pieces and we still have them at home. She was very dependable, she was always there for us. There was nothing that she wouldn't do for us."

The U.S. Embassy in Mexico confirmed on Twitter that "an American citizen has been killed."

Miguel Angel Pech, attorney general for the Mexican state of Quintana Roo, said Monday that four male victims died from gunshot wounds and one woman appeared to have been killed in an ensuing stampede as club-goers fled the shooting.

Four Americans were also injured, Pech said. Among them were Ignacio Valencia of Houston and Michael Angel Palenque of New York.

Valencia, who was shot in each hand, said his group of friends had met up with Villanueva, the teenager who died. He told NBC News about her final moments.

"We're sitting outside and we see a massive amount of people running towards us ... I hear bullets," he said. "As soon as it happened everyone said, 'Get down! get down!' So I told Alejandra get down. So we're getting down and all she's telling me is, in Spanish, that she doesn't feel anything, she doesn't feel anything. And I said, 'Well, just stay with me.'"

He said Villanueva soon fell unconscious.

"I'm panicking. I'm trying to wake her up. I'm telling her, 'Let's go, we have to go,'" he said. "I'm screaming for my other friend, Michael, who was there with us as well ... That's when my friend comes and we're trying to wake her up and everything, but she's not responsive. I kind of felt that she was no longer with us at that point."

At that point the police arrived. Knowing the officials were now handling the situation, he and his friend ran and found an ambulance.

He described it as "the worst experience I've ever had in my entire life."

NBC affiliate KHQ reported that a Washington state woman was among 15 people who were wounded but survived the shooting.

The victims also included two Canadians, one Italian and one Colombian, according to Mexican officials

On Monday, the event's organizers said three of those killed were of a part of BPM's security team.

"We are overcome with grief over this senseless act of violence and we are cooperating fully with local law enforcement and government officials as they continue their investigation," according to a statement posted on The BPM Festival's Facebook page.